What was the name of the early U.S. flag that retained the British Union in its canton?

The early U.S. flag that retained the British Union in its canton was the Grand Union Flag.

The flag used thirteen alternating red and white stripes, representing the thirteen colonies, while its upper corner displayed the British Union Flag. It was used by the Continental Army and Continental Navy before the Stars and Stripes design was adopted.

The Grand Union Flag is associated with George Washington's headquarters during the Siege of Boston and was raised at Prospect Hill on January 1, 1776. It expressed the colonies' military organization while still visually retaining a connection to Britain.

The name “Grand Union Flag” became common later; contemporaries used several names, including the Continental Colours. It should not be confused with the 1777 flag, whose canton contained stars rather than the British Union.